Polyurethanes have found widespread use in recent years as liners for fuel tanks and other liquid chemical containers. These polymer materials provide effective, economical liquid barriers which are essential in maintaining the integrity of storage tanks. Over time, however, environmental factors, such as heat, ultraviolet light, or radiation exposure can cause degradation of the polyurethane material and, thus, a breach of the containment barrier.
Polymer degradation is often accompanied by a minor change in the material's color, as well as changes in many of its other physical characteristics. In cases of extreme degradation, the color change may be significant enough to be detected by visual inspection of the material. However, potentially dangerous degradation may occur long before the material exhibits a significant change in its visual characteristics. Moreover, certain materials, such as black polyurethane, may undergo shifts in color which are very difficult to detect by visual inspection or standard color measurement techniques.
For the reasons discussed above, it is apparent that there is a need for an effective inspection system for detecting the degradation of polymer-based materials which might not be apparent from a visual inspection of the material. The present invention overcomes the difficulties caused by standard color comparison techniques by providing an optical inspection system employing laser-induced fluorescence, as is discussed in greater detail below.
In order to understand the principles of operation of the present invention, it is important to understand the meaning of luminescence, as well as the historical evolution of the definition of luminescence. Historically, materials were said to exhibit characteristics of "luminescence" if they emitted photons after being irradiated with light having a wavelength in the range of approximately 1800 to 3700 Angstroms (ultraviolet). Prior art definitions of this phenomenon have included two categories: fluorescence and phosphorescence. A material was said to exhibit fluorescence if the luminescence ceased after termination of the irradiation. However, if the luminescence persisted after irradiation, the phenomena was termed phosphorescence.
The above-mentioned definitions evolved at a time when observations of the persistence of luminescence were made with the unaided eye. The development of sophisticated instruments capable of measuring the persistance of luminescence for very short time periods, e.g., nanoseconds, has led to a more precise definition of the above-mentioned terms and has changed the definition of luminescence for some materials. For example, it is now known that many materials which have been characterized in the literature as being fluorescent emit luminesence for as long as 1000 microseconds after termination of excitation. This luminescence offers significant information regarding the physical characteristics of the illuminated material and in the present invention can be used to detect deterioration of the material, as will be discussed in greater detail below.
It is well known that certain materials luminesce in the presence of ultraviolet or blue light and that the variation of the visible light luminescence can be used to determine certain features of the material. An example of an apparatus for using these phenomena to detect the presence of caries in human teeth is shown in U.S. Pat. Nos. 4,290,433 and 4,479,499 issued to Alfano. The luminescence in human teeth which is essential to the methods shown in these patents is dependent on the recognition of total visible luminescence. Further, the detection of the caries as shown therein relies on a visual recognition of differences in the color of the reradiated light from the teeth. While this luminescence technique is useful for detecting certain types of characteristics of materials, it is not suitable for an application such as that shown in the present invention because the technique is dependent on visual recognition of color differences in the luminescence of the material.
The invention method overcomes the shortcomings of previous optical inspection systems because it takes advantage of complex excitation-luminescence spectra of polyurethanes. Thus, two samples which both reflect approximately the same spectrum can have different fluorescence charactersitics which can be differentiated to distinguish between various grades of the material.